Satellite navigation systems have been designed to allow the location and the following of trajectories of users on the ground, or at the very most of aircraft flying at low altitude. However, it is also known to use them to follow the orbit of space vehicles, mainly in low orbit (LEO, standing for “Low Earth Orbit”), much more rarely in the case of intermediate orbits (MEO, standing for “Medium Earth Orbit”) or geostationary orbits (GEO). However, these systems have not been used hitherto to follow the transfer or orbital placement phases, since the usage constraints are strong: the GNSS signal is generally weak since these orbits are outside the main lobes of the transmitting antennas of the GNSS satellites (which are logically pointed towards the earth); there are significant periods of non-availability; the activation of the propulsion gives rise to significant location errors and prevents recourse to the simplifying assumption according to which the vehicle follows a Keplerian orbit, etc. These constraints are still more significant in the case of space vehicles employing electric propulsion (emission of ions, for example), wherein the thrust is continuous over the whole of the duration of the transfer phase.
For these reasons, orbital positioning in the course of transfer is carried out by virtue of measurements of distance on a TMTC communication channel between the space vehicle and terrestrial stations. This solution exhibits numerous drawbacks: need to use several ground stations to have sufficient geometric diversity, thus giving rise to significant cost; location inaccuracy; lag between the measurements which entails large drifts; low autonomy in case of degradation of the communication link.
The invention is aimed at remedying the aforementioned drawbacks of the prior art. More particularly, it is aimed at enabling GNSS following of the transfer orbits and phases of orbital placement of space vehicles, notably using continuous thrust and more particularly electric propulsion.
In accordance with the invention, such an aim is achieved by virtue of tight coupling between a GNSS processing and a model for estimating a state vector of the space vehicle taking into account the propulsion of the latter.